1. Field of the Invention
This invention relates generally to power supply circuits and more particularly is directed to improvements in a power supply circuit of the type in which a switching element is pulse-width modulated.
2. Description of the Prior Art
In the prior art, it has been proposed to employ a chopper system as a kind of power supply circuit. This prior art power supply circuit employing a chopper system generally comprises a power switch connected to a commercial power source, a power source filter, a full-wave rectifier, means for chopping the output from the rectifier, a transformer and a rectifier. More particularly, the DC voltage from the full-wave rectifier is supplied to a series connection of the primary winding of the transformer and a transistor for chopping or switching, and a PWM (pulse width modulated) pulse from a PWM modulator is supplied to the transistor to switch it ON and OFF. Thus, an AC voltage is induced on the secondary winding of the transformer, and such AC voltage is fed to the rectifier to be rectified as a desired DC voltage which is delivered to an output terminal. In order to stabilize the output DC voltage delivered to the output terminal, the output DC voltage is fed through a variable resistor to a transistor which compares the output DC voltage with a reference voltage from a constant voltage diode. The compared output is supplied, for example, through a photo-coupler and an amplifier, to a modulator for modulating a carrier received from an oscillator. Thus, the pulse width of the PWM pulse from the modulator is varied in response to the DC voltage at the output terminal and hence the DC voltage at the output terminal is stabilized at a constant value. Further, during operation of the power supply circuit, a DC voltage from the full-wave rectifier is fed through a transistor to the modulator and associated amplifier and oscillator as the operating voltage therefor.
With the above described power supply circuit employing a chopper system, the input AC side and the load side are separated with each other by the transformer and photo-coupler. Moreover, since the transistor carries out the switching operation, the power supply circuit theoretically has a high efficiency.
In practice, however, when the commercial AC voltage is 100 V, the DC voltage from the full-wave rectifier is about 140 V, whereas the operating voltage of the modulator and associated amplifier and oscillator is about 6 to 15 V. Accordingly, it is necessary that the DC voltage from the full-wave rectifier be dropped to the level required for the operating voltage, for example, by a transistor, resistor and so on, so that the actual efficiency of the power supply circuit is reduced. Further, the transistor employed has to be capable of withstanding a high voltage with the result that the reliability of the circuit is lowered and the circuit becomes expensive.
In order to avoid the above problems, it has been proposed to add a tertiary winding to the transformer, and to feed an AC voltage induced across the tertiary winding to a rectifier for producing a relatively low DC voltage as the operating voltage for the modulator and associated amplifier and oscillator.
However, in the last described power supply circuit, when the power switch is initially turned ON, the chopping transistor is OFF and hence the rectifier connected to the tertiary winding produces no DC voltage. Thus, the modulator and associated amplifier and oscillator are inoperative, and the modulator produces no PWM pulse so that the chopping transistor is maintained in its OFF-state. In other words, even if the power switch is turned ON, the power supply circuit as so far described is not made operative.
In order to solve that problem, the existing power supply circuit is provided with a start circuit to supply the DC voltage from the full-wave rectifier to the modulator and associated amplifier and oscillator as the operating voltage thereof for several seconds from the time when the power switch is first turned ON. However, due to the need for the start circuit, the existing power supply circuit is relatively expensive.